Correct an unsound optimization in event dispatch.

components/script/dom/eventdispatcher.rs

@@ -112,19 +112,23 @@ pub fn dispatch_event(target: &EventTarget,
     assert_eq!(event.phase(), EventPhase::None);
     assert!(event.GetCurrentTarget().is_none());
 
+    // Step 1.
+    event.mark_as_dispatching();
+
     // Step 2.
     event.set_target(target_override.unwrap_or(target));
 
     if event.stop_propagation() {
         // If the event's stop propagation flag is set, we can skip everything because
-        // it prevents the calls of the invoke algorithm in the spec and we asserted
-        // at the beginning that steps 10-12 don't need to be executed.
+        // it prevents the calls of the invoke algorithm in the spec.
+
+        // Step 10-12.
+        event.clear_dispatching_flags();
+
+        // Step 14.
         return !event.DefaultPrevented();
     }
 
-    // Step 1. Postponed here for the reason stated above.
-    event.mark_as_dispatching();
-
     // Step 3. The "invoke" algorithm is only used on `target` separately,
     // so we don't put it in the path.
     rooted_vec!(let mut event_path);

tests/wpt/web-platform-tests/dom/events/Event-propagation.html

@@ -11,14 +11,16 @@
   test(function() {
     var called = false;
     var callback = function() { called = true };
+    this.add_cleanup(function() {
+      document.head.removeEventListener("foo", callback)
+    });
     document.head.addEventListener("foo", callback);
     document.head.dispatchEvent(ev);
     assert_equals(called, expected, "Propagation flag");
     // dispatchEvent resets the propagation flags so it will happily dispatch
     // the event the second time around.
     document.head.dispatchEvent(ev);
     assert_equals(called, true, "Propagation flag after first dispatch");
-    document.head.removeEventListener("foo", callback);
   }, desc);
 }